Line impedance is an important measurement which is used for evaluating power quality and other purposes. Line impedance refers to the impedance of an electrical line which carries electrical current at a predetermined line frequency. It is important to know line impedance of a power line in order to determine the effect of a load on line voltage distortion. In particular, all nonlinear harmonic-producing loads draw a distorted line current that interacts with line impedance to produce line voltage distortion.
In view of the above, many line impedance measurement techniques have been developed. Unfortunately, each may suffer from a number of limitations. For example, in one technique, the assumed number of elements and topology for a lumped equivalent circuit of the line impedance is duplicated in a variable impedance. A bridge is created with the variable impedance tuned to balance the line impedance. See L. E. Banta et al., "Distribution System Impedance Measurement Technique", Proceedings of the International Conference on Harmonics in Power Systems, Worcester Polytechnic Institute, October, 1984, pages 220-224. Another technique derives impedance by measuring the line voltage, magnitude and phase before and after applying a known load. See D. Crevier, "Estimation of Higher Frequency Equivalent Impedances by Harmonic Analysis of Natural Waveforms", IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 2, March/April 1978, pages 424-431.
In another technique, damped oscillatory voltage and current waveforms caused by transients such as capacitor switching, transformer inrush and/or line energization and deenergization are used to estimate line impedance. Spectral estimation may be used to estimate line impedance, as described in the publication by A. A. Girgis et al., "Frequency Domain Techniques for Modeling Distribution or Transmission Networks Using Capacitor Switching Induced Transients", IEEE Transactions on Power Delivery, Vol. 4, No. 3, July 1989, pages 1882-1890. Alternatively, quadratic-impedance-model fitting may be used to estimate line impedance, as described in the publication by M. M. Forti et al., "Power Line Impedance and the Origin of Low-Frequency Oscillatory Transients", IEEE Transactions on Electromagnetic Compatibility, Vol. 32, No. 2, May 1990, pages 87-97. Unfortunately, these transients may not sufficiently excite the line impedance at all frequencies.
In yet another technique, the measured response of an injected white noise signal has been used with spectral estimation to obtain a measure of line impedance magnitude without phase. See the above Banta et al. publication. Finally, injected sinusoidal currents have been used to measure line impedance in the 5-100 kHz range. See the publication by R. M. Vines, "The Characterization of Residential Impedances and Noise Sources for Power Line Carrier Communications", Center for Communications and Signal Processing, North Carolina State University, Raleigh, N.C., Jun. 10, 1983. Unfortunately, the above techniques may be limited in accuracy and/or flexibility.
A technique for measuring impedance of a deenergized line in order to evaluate the condition of electrical insulation in a high voltage power system, is described in published European Patent Application No. 0 600 726 A2 to Allfather entitled "Impedance Measuring". In this technique, a test voltage signal of frequency different from the line frequency by an increment significantly less than the line frequency is provided to the deenergized line in order to provide an impedance measurement signal representative of the impedance of the power system at the test frequency. Unfortunately, this technique is applicable to a deenergized line onto which a test voltage signal can be impressed.